Many developing countries have begun to investigate the possibility of providing telephone service to a larger percentage of their respective citizenry. In many developing countries the number of telephones per person (telephone density) in rural regions is typically only one-fifth to one tenth of the telephone densities characteristic of urban areas. For example, telephone densities in China's urban centers typically exceed 30 telephones per 1,000 population, while telephone density in China's rural areas is roughly 4 telephones per 1,000 population.
In order to improve service to rural areas many developing countries have attempted to identify telephone systems enabling cost-effective switching, transmission, and distribution suitable for the low capacity requirements of rural installation. At the same time, however, it is desired that the system be expandable to accommodate higher traffic, a larger subscriber base, and allow for advanced billing and data communication functions to be supported as rural areas develop.
Radio equipment has been used, particularly in rural areas, for providing telephone service to a distributed set of users. The highly variable distribution requirements of the rural networks in developing countries has spurred efforts to develop cost effective radio telephone systems. Radio transmission offers several advantages relative to communication facilitated by satellite or cable networks. For example, it may be impractical to bury cable in regions having uncultivatable (e.g., rocky or hardpan) soil, and aerial cable has proven to be relatively costly to install and maintain. In contrast, the cost of microwave radio links is relatively insensitive to distance due to the extended coverage capability afforded by repeater stations.
Satellite telephone systems are often used to service remote rural areas, or to provide service over wide geographic regions of diffuse subscriber concentration. However, a minimum number of subscribers is required to justify the significant investment required by the provision of satellite base stations and related facilities. Low capacity radio links can be a less expensive alternative to satellite systems when providing service to remote areas (e.g., islands) with relatively low levels of telephone traffic. Moreover, radio links can meet the transmission demands of low density demand pockets, particularly those located off of larger demand routes.
In trunked radio communication systems a number of radio users (subscribers) share a group of communication channels, where typically each user will require a channel for only a small percentage of the time. A base station serves as an exchange between the subscribers and a set of dedicated lines, i.e., trunks, from the public network. Subscribers may use mobile or portable two way and fixed base communication modules, with radio repeaters generally being used to provide the communication channels over an extended range. Conventional repeaters do not perform any call switching functions, but rather merely relay information signals linearly between subscribers and the base station. It follows that when one subscriber desires to communicate with another located nearby, the call must nonetheless be transmitted to the base station and rebroadcast to the intended recipient subscriber. Accordingly, a radio telephone system operative to provide localized call routing among proximately located subscribers would allow a base station of a given traffic capacity to service a larger number of subscribers.
Each repeater unit within trunked systems typically receives communication signals from subscriber units on a first path, and rebroadcasts the signals at higher power on a second path to other subscriber units. Trunking systems generally use different assigned frequencies for these simultaneous paths. Each pair of frequencies assigned to a repeater unit constitutes one of the several communication channels of the trunked system. Channels carrying voice and other message communications are known as information channels, those carrying signal and control information are known as control channels.
In trunked systems it is often necessary that a user desiring to communicate first request permission to access the group of channels and then wait for permission and for an assignment to a particular channel. In transmission-trunking systems a user retains his channel assignment for only a single transmission, while in message-trunking systems the channel is retained until an entire message comprising multiple transmissions is completed. Because trunked systems involve sharing resources, subscribers must often wait for availability of information channels. The trunk controller responds to a request for channel with a busy signal and notifies the subscriber when a channel later becomes available. Upon notification of channel availability the subscriber may be allowed only a brief response interval to claim the assigned channel. This procedure is not only inconvenient, but also leads to subscriber access delays and to the consumption of communications resources during the response interval.
Access to the public telephone network from a trunked system requires a call origination procedure. One procedure often used is for a radio subscriber to place a request for a channel capable of providing interconnection to the public network. When such a channel becomes available, the controller grants permission to use it and generates a dial tone. The radio subscriber originates the call by transmitting tone or data signaling information, which couples to the public network. The radio subscriber hears busy tones or ringing tones and eventually becomes connected with the requested party or terminates his call attempt.
In conventional trunked systems the inbound and outbound frequencies associated with a given channel are typically assigned simultaneously and maintained for the duration of a conversation. This is so even though the radio subscriber typically cannot make use of them simultaneously. That is, while transmitting the radio subscriber does not hear, and cannot be interrupted, by a party from the public network. Such unnecessary assignment of paired frequencies removes resources from the collection of frequencies available to interconnect calls.
Many existing trunked radio telephone systems are designed to be in conformance with standards derived from recommendations made by the International Telecommunications Union (ITU). One objective of the ITU is to promote adherence to a universal set of equipment interface conventions. Widespread compliance with the body of recommendations promulgated by the ITU enables interoperability between radio and telephone equipment, particularly when crossing of international boundaries is involved. Under the ITU recommendations standard interfaces are to be provided for each manufactured equipment component. Hence, radio telephone systems are generally composed of a collection of "stand-alone" components, each disposed to communicate using a prescribed public domain protocol. Unfortunately, the need for standard interfaces to facilitate compatibility between stand-alone components tends to increase the cost of the radio telephone networks in which such components are incorporated.